The communications industry is in a race for high-speed data transmission. The systems being installed are leading edge technology designed to allow access to the data information highway at incredible new speeds.
Everyone is familiar with the Internet and the connection of a computer or a television via a modem to the Internet through telephone lines. Modems can only transmit or receive as fast as is allowed by the lines connecting the modem to a communication method. Most modems have a maximum speed at this time of 56,000 bits per second but, within 10 years, the transmission and receiving speeds will be from 10 Megabits per second to Gigabits per second. This will allow incredible amounts of data and information to be transferred in very short periods of times and will open a communications revolution similar to the industrial revolution in the previous century.
There are five basic technologies that are or will be used for data transmission and each will have its own use or market for communications, depending upon the circumstances of the installation. The five major technologies presently available for data information are low level orbiting satellites, telephone data links, cable television, wireless broadband systems, and ground-based broadband systems. These systems could be used individually or could be combined into a system that would utilize the best of each technology.
High-level orbiting communications satellites are in place now, usually as a geosynchronous orbiting satellite or, in other words, a satellite that is in a fixed orbit at a speed that matches the speed of the earth. The drawback of the use of these satellites for high-speed data transmission is the time it takes to transmit the data to the satellite and then back to earth as a line of sight transmission. One way to reduce the transmission and receiving time is to reduce the orbiting height of a data satellite. This is being contemplated by a number of communications companies. These low-level orbiting satellites may be in place sometime in the near future.
Telephone cables have been distributed throughout the world as a means of voice transmission from point to point. Within the past ten years, these same voice transmission cables have been used as a means of interconnecting computers and fax machines by acting as a data link. Instead of transmitting and receiving just voice communications, these same lines are now transmitting analog and digital signals. These signals can be affected dramatically by the dc resistance and ac impedance of the cable and may require amplifiers or boosters that are able to handle the high frequencies used in data transmission. Many of these telephone cables have now been replaced with optical fiber cable to a central distribution point so the impedance and the resistance would only be a problem for the hard-wired portion of the system. The use of optical fiber cable also allows many different frequencies and signals to be transmitted simultaneously across the cable with a minimum loss of signal.
A cable TV system is actually a form of closed circuit TV and is usually connected using a hard-wire cable from the source or distribution point to the user of the signal. Cable TV systems normally require a modem or converter box to convert the signal from the cable to a signal that can be used in the TV. Cable TV systems are also being converted from strictly hard-wired systems to a combination of hard wire, optical fiber cable, and, at times, satellite reception units. These combination systems allow feed to a local area distribution point within a neighborhood using optical fiber cable and then using a hard-wired system for distribution to each building or user. This allows the system to transmit and receive many different signals and frequencies with a minimum loss of signal. There are even some systems that use a combination of hard wire, optical fiber cable, and microwave transmission and reception to provide the data link to a customer.
Broadband can be either wireless or a hard-wired system or a combination of both. The concept of broadband is not new and has been used in radio transmission systems for a long time. However, a network-powered broadband communications system for general use is relatively new. A network-powered broadband system is one that provides any combination of voice, audio, video, data, and interactive services through a network interface unit by a signal provided by a communications company. These various signals originate as separate signals and then are combined at a distribution point and transmitted to the neighborhood or building as a combined signal. At the building interface unit, the signal is split back apart and distributed to the individual loads within the building. These loads could be for television, sound systems, connecting computers to the Internet, video conferencing systems, security and fire alarm systems, or just about any interactive system.
Article 830 of the 1999 NEC(covers hard-wired network-powered broadband system installations in both low power and medium power levels for both outside and inside the building. The low power network broadband systems have a circuit voltage limitation of up to 100 volts, a maximum power limitation of 250 VA, with a maximum current limitation of 1000 ( Vmax. The maximum current for a 100-volt system would be a 10 amperes. Medium power network broadband systems have a maximum circuit voltage of 150 volts, a maximum power limitation of 250 VA, with a maximum current limitation of 1000 ( Vmax. The maximum current for a 150-volt system would be a 6.67 amperes.
A typical basic hard-wired network-powered broadband system configuration includes a cable supplying power and broadband signal to a network interface unit that converts the broadband signal to the component signals. Typical cables used in the installation of broadband are coaxial cables with both broadband signal and power on the center conductor, composite metallic cables with a coaxial member for the broadband signal and a twisted pair for power, and composite optical fiber cable with a pair of conductors for power. Larger systems may also include network components such as amplifiers that require network power.
As can be seen, the electrical industry and the National Electrical Code( are continuing to respond to new and innovative ways of communicating. We can all look forward to the new millenium with assurance that these new systems will continue to be installed in a safe and workmanlike manner.